Concept: A molecule possesses a permanent dipole moment if it has a net separation of positive and negative charges. This occurs when there are polar bonds and the molecular geometry is such that the individual bond dipoles do not cancel each other out. The presence of lone pairs on the central atom often leads to an asymmetrical geometry and thus a net dipole moment.

Why (B) SF₄ is correct:

1. SF₄: Sulfur (S) is the central atom. It has 6 valence electrons. Four electrons are used to form bonds with four fluorine (F) atoms, leaving 2 electrons as one lone pair. Thus, the steric number is 4 (bonds) + 1 (lone pair) = 5. According to VSEPR theory, a molecule with 5 electron domains and one lone pair adopts a see-saw geometry. This geometry is asymmetrical, and the bond dipoles (S-F bonds are polar) do not cancel out, resulting in a net permanent dipole moment.

Option Analysis:

• A) BF₃: Boron (B) is the central atom with 3 valence electrons, forming 3 bonds with F atoms. There are no lone pairs. The geometry is trigonal planar, which is symmetrical. The three B-F bond dipoles cancel each other out, so BF₃ has no net dipole moment.
• C) SiF₄: Silicon (Si) is the central atom with 4 valence electrons, forming 4 bonds with F atoms. There are no lone pairs. The geometry is tetrahedral, which is symmetrical. The four Si-F bond dipoles cancel each other out, so SiF₄ has no net dipole moment.
• D) XeF₄: Xenon (Xe) is the central atom with 8 valence electrons. Four electrons are used to form bonds with four F atoms, leaving 4 electrons as two lone pairs. Thus, the steric number is 4 (bonds) + 2 (lone pairs) = 6. According to VSEPR theory, a molecule with 6 electron domains and two lone pairs adopts a square planar geometry. This geometry is symmetrical, and the bond dipoles cancel each other out, so XeF₄ has no net dipole moment.

Correct Answer: (B)